Previously it was necessary to create a new Sonic instance every time the
processor was flushed. Add a flush() method to Sonic so that it can be reused
when seeking. It still needs to be recreated when parameters change.
SonicAudioProcessor and SilenceSkippingAudioProcessor have methods for setting
parameters that are documented as taking effect after a call to flush(), but
actually the value returned by isActive() was updated immediately. Track the
pending values and apply them in flush() to fix this.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=191442336
f2399c1c85